Existing ring main units generally use gas insulation, i.e., air insulation or SF6 gas insulation. That is, the main circuit mounted in the insulating cylinder of ring main unit (RMU) adopts air or SF6 gas as the insulation media. Due to its poor insulation and safety, the air insulation ring main unit has gradually become unadaptable to the requirements of the technology development. SF6 gas ring main unit, compared to the air ring main unit, greatly improves insulation and safety due to its small volume, full insulation, and full sealing, and thus is widely used in city power grid. However, SF6 is widely regarded as one of greenhouse gases, and its greenhouse effect is 2500 times of CO2. Further, SF6 can exist in the atmosphere up to 3400 years, and can produce toxic gas fluoride (e.g. SF4, S2F10) under high voltage arc environment, so as to severely pollute the environment. Further, in view that SF6 can produce toxic gas under high voltage arc environment, to prevent leakage, the SF6 gas RMU is generally required to have high sealing and complex processing, which leads to the unit to have high cost and complex maintenance and test, such that the cost of manufacturing and maintenance is quite expensive. Furthermore, the charged body is exposed to SF6 gas, which is easy to be eroded, leading to degradation of insulation level and easily to cause casualty/accidents.
Solid insulated vacuum ring main unit is considered to be the best choice to replace the gas insulated ring main unit due to its good insulation and stability, pollution-free, simple manufacturing and maintenance, and low cost. However, the switch in the existing solid RMU usually adopts two-position (i.e., working, isolation, ground) manner, and the main isolation is load-side isolation. During servicing, the main bus-bar must be disconnected, which reduces reliability of power supply. Further, it is difficult for the maintenance staff to confirm position of the switch, which may cause security risk.
Thus, there is a need for a novel insulating cylinder and a main circuit structure using such insulating cylinder, to facilitate maintenance of the switchgear and to have visible position indicator to ensure maintenance personnel's safety. Further, three-position manner (i.e., working, isolation and ground) is used to improve the insulating property of the product, to further improve safety of the product.